Total knee arthroplasty (“TKA”) is a procedure for treating an injured, diseased, or worn human knee joint. In a TKA, an endoprosthetic joint is implanted, replacing the bearing surfaces of the joint with artificial members. Proper alignment of the joint and substantially equal tension in the soft tissues surrounding the joint are important factors in producing a good surgical outcome.
A human knee joint “J” is shown in FIG. 1-4. The joint J is prepared for implantation by cutting away portions of the femur “F” and the tibia “T”. FIGS. 1 and 2 show the joint in extension, with cutting planes for a tibial cut 1 and a distal femoral cut 2. The tibial cut 1 and the distal formal cut 2 cooperate to define an extension gap “EG”. FIGS. 3 and 4 show the joint J in flexion, with cutting plane 3 for a posterior cut. The tibial cut 1 and the posterior cut 3 cooperate to define a flexion gap “FG”.
A goal of total knee arthroplasty is to obtain symmetric and balanced flexion and extension gaps FG, EG (in other words, two congruent rectangles). These gaps are generally measured in millimeters of separation, are further characterized by a varus or valgus angle measured in degrees, and are measured after the tibia cut, distal femoral cut, and posterior femoral cut have been done (to create flat surfaces from which to measure). It follows that, to achieve this balance, the ligament tension in the lateral and medial ligaments would be substantially equal on each side, and in each position; it also follows that the varus/valgus angle in flexion and extension would be 0°.
Some surgeons favor the use of a measured resection technique in which bone landmarks, such as the transepicondylar, the anterior-posterior, or the posterior condylar axes are used to determine proper femoral component rotation and subsequent gap balance. Others favor a “gap balancing technique” in which the femoral component is positioned parallel to the resected proximal tibia with each collateral ligament substantially equally tensioned to obtain a rectangular flexion gap.
One problem with prior art balancing techniques is that it is difficult and complex to achieve the proper balance. Current state-of-the-art gap balancing devices do not enable balancing with the patella in-place and are large, overly-complicated devices that work only with their respective knee systems.